Examples of various compounds, conventionally known as emulsifiers or solubilizers include: ethylene oxide-based non-ionic surfactants such as a polyoxyethylene alkyl ether, a polyoxyethylene polyhydric alcohol/fatty acid ester, and a polyoxyethylene alkyl phenyl ether; and food surfactants such as a sorbitan/fatty acid ester, a sucrose/fatty acid ester, and a fatty acid ester of a polyglycerol (including a polyglycerol ester of interesterified ricinoleic acid).
Of those, the fatty acid ester of a polyglycerol is one of the most useful surfactants because the fatty acid ester of a polyglycerol ensures safety to a human body and to the environment, can have diverse compositions, and has high versatility. The fatty acid ester of a polyglycerol is produced by polymerizing glycerol as one of raw materials at high temperatures in the presence of an alkali catalyst such as caustic soda, deodorizing and decolorizing the resultant to obtain a polyglycerol reaction product, and subjecting the polyglycerol reaction product and a fatty acid as raw materials to an esterification reaction.
Meanwhile, known processes for industrial production of a polyglycerol used as a raw material are as follows:
(i) a process for producing a polyglycerol through recovery from a distillation residue of glycerol;
(ii) a process for producing a polyglycerol through dehydration condensation of glycerol;
(iii) a process for producing a polyglycerol through direct polymerization of epichlorohydrin, hydrolysis, and then dechlorination; and
(iv) a process for producing a polyglycerol through addition of glycidol to glycerol or a polyglycerol in the presence of an alkali catalyst such as NaOH or amines, or an acidic catalyst such as acetic acid.
However, analysis of a composition distribution of the polyglycerol reaction product obtained through each of the above processes by liquid chromatography/mass spectrometry as described below confirms that produced through intramolecular dehydration are not only cyclic low molecular compounds but also a polyglycerol having a cyclic structure in considerably high content with a molecular weight ranging from several hundreds to several thousands, which are formed through removal of one to several water molecule(s) from a polyglycerol molecule.
The inventors of the present invention have found that a polyglycerol containing the above-described polyglycerol having a cyclic structure in high content has inhibited properties in hydrophilicity and the like. The inventors of the present invention have also found that deterioration of properties such as water solubility, surface activity, and the like is caused in a fatty acid ester of a polyglycerol obtained from the above-described polyglycerol and a fatty acid ester.
As a polyglycerol produced through a reaction, a reaction product obtained by reacting glycerol with epichlorohydrin or glycidol, or by reacting glycerol or a polyglycerol with epichlorohydrin, monochlorohydrin, dichlorohydrin, or glycidol has been heretofore used as it is or used after having been optionally purified.
Example of a purification process include: purification by heating under reduced pressure of several Torr in a stream of a gas such as nitrogen or water vapor for deodorization or removal of unreacted raw materials; purification by removing ionic components such as a catalyst used with an ion-exchange resin, an ion-exchange membrane, or the like; purification by removing color components or odor components using an absorbent such as active carbon; and purification by reduction treatment through hydrogenation or the like.
However, in the purification processes for a polyglycerol reaction product, the composition distribution of the resulting a polyglycerol reaction product has not been discussed in detail. Regarding a polyglycerol reaction product obtained by polymerizing the most generally used glycerol at high temperatures in the presence of an alkali catalyst such as caustic soda, and deodorizing and decolorizing the resultant, cyclic compounds having a low molecular weight such as cyclic diglycerol have been discussed extensively. However, actually, analysis on the composition distribution of a polyglycerol having a molecular weight in the range of several hundreds to several thousands has been hardly discussed in spite of a fact that a dehydration reaction is known to take place in a polyglycerol molecule.
Commercially distributed a polyglycerol is called tetraglycerol, hexaglycerol, or decaglycerol according to an average polymerization degree calculated from a hydroxyl value. However, in fact, the polyglycerol is a mixture of various glycerol polymers each having a polymerization degree from 2 to 10 or more, and may include unreacted glycerol (polymerization degree of 1).
It has been revealed that a polyglycerol of a relatively low molecular weight such as glycerol, diglycerol, or triglycerol may exist in high content in a polyglycerol containing such a mixture of various glycerols in order to adjust the average polymerization degree calculated from hydroxyl values to the same value.
JP-B 5-1291 (claim 1, lines 12-22 in the 3rd column, and Examples) discloses slightly colored polyglycerol having a large polymerization degree obtained by: adding a phosphoric acid catalyst to glycerol or a polyglycerol; and subjecting the resultant to an addition reaction with glycidol at 115 to 125° C.
JP-A7-216082 (claim 1, paragraph 0008, and Examples) discloses a process for producing a polyglycerol in which glycerol is polycondensed in a boiling state of a reaction mixture at 200 to 270° C. in the presence of an alkali.
JP-A 2002-30144 (claim 1, and Examples 1 to 12) discloses a process for producing a polyglycerol in which glycidol alone is added and allowed to react in the presence of an alkali metal halide without the use of glycerol as an initiator.
Regarding a fatty acid ester of a polyglycerol, JP-A 7-308560 [claim 1 and Comparative Example 1 (Production of a polyglycerol used in each Example)] discloses a process in which a polyglycerol obtained by distilling off low molecular weight polymers from a polyglycerol produced through polycondensation of glycerol at 240° C. in the presence of sodium hydroxide is allowed to react with a fatty acid.
Alternatively, JP-A8-109153 (claim 1 and Examples) discloses a process for producing a fatty acid ester of a polyglycerol obtained through an addition polymerization reaction of glycidol with a fatty acid. However, a product obtained through this process is restricted to a polyglycerol/mono-fatty acid ester alone.